ST. IGNACE — Canadian oil transport giant Enbridge, the U.S. Coast Guard and several other federal, state and local agencies took to the waters of the Great Lakes Thursday in boats big and small, testing their preparedness and capabilities to contain what many consider as the worst of nightmare scenarios for the Great Lakes: a leak in Enbridge's Line 5 pipeline that runs along the bottom of the Straits of Mackinac.

The line includes twin, 62-year-old pipelines at the bottom of the Straits, through which Enbridge transports light crude oil and other petroleum products between Michigan's peninsulas. Many fear that a rupture on Line 5 similar to Enbridge's Line 6B oil pipeline rupture in Marshall in July 2010 could devastate a wide swath of the northern Great Lakes, harm island and shoreline communities and their water supplies, as well as damage Michigan's $7.4-billion boating and fishing industry.

Concern about a spill in such a critical area of the Great Lakes was heightened by a report from University of Michigan researchers that showed a spill there could be catastrophically far reaching, prompting state officials to form a committee to study what could be done pre-emptively to ward off a disaster. That committee recently reached an agreement with Enbridge to make sure heavy crude — the substance involved in the Marshall spill — would never be transported through Line 5. The company notes it has never transported heavy crude through Line 5, nor does it plan to do so.

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Enbridge officials say the chances of a spill on Line 5 are slim. Still, the company along with the other local, state and federal agencies wanted to show what they were prepared to do in the event the unthinkable were to happen.

Thursday's exercise included about 24 boats, from the Coast Guard cutter Alder deploying its spilled oil removal system to "vessels of opportunity," tugs and barges in the Straits area put into service to pull booms, the floating, net-like spill-containment devices, said Steven Keck, the Coast Guard's oil spill contingency specialist based in Sault Ste. Marie. Shoreline protection booms also were deployed near St. Ignace; the imaginary spill funneled into small bays and inlets around the area's little islands for collection by rotating, squeegee-like skimmers that move the oil into vacuum trucks onshore.

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Helicopters and drones from Enbridge and other responding agencies patrolled the skies overhead.

"This is pretty much what you'd see after the first 24 hours," Keck said. "If the incident went beyond that, a lot more assets would be coming in."

But the staged response drill was of little comfort to some residents who live nearby. Mackinaw City resident Anabel Dwyer rejected the premise of Thursday's exercise. It's not about showing how well Enbridge and responders can address a burst pipeline, she said.

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"We must not have a spill," she said. "A spill would be catastrophic. This section of the line should be closed, and before another winter."

Charles Usher, president of Detroit-based Marine Pollution Control, is Enbridge's contracted oil spill responder in the Straits and elsewhere. While some of the response equipment needed to address a Straits oil spill is kept in the area, much would have to come from Detroit, a nearly five-hour drive, he said.

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A spill would likely make its way to shoreline areas along the northern tip of the Lower Peninsula — and that's helpful for Usher's purposes, he said.

Environmental advocacy groups were quick to point out that Thursday's exercise was planned for months, with necessary boats and equipment convening on a schedule — far from the scenario of a real spill, in which key containment equipment would take hours to arrive. The Alder, for example, is docked in Duluth, Minn., more than 400 miles from the Straits.

The Straits of Mackinac, the stretch of water where Lakes Michigan and Huron converge and over which the Mackinac Bridge spans, was called by University of Michigan research scientist David Schwab "the worst possible location for an oil spill on the Great Lakes." The area's complex water currents can move in different directions at different lake depths, and can change from one direction to another in a matter of hours.

Twenty-four hours into a Straits oil spill, "you can be all the way from Rogers City to Beaver Island. The affected area is enormous," said Mike Shriberg, executive director of the National Wildlife Federation's Great Lakes Regional Center.

The nearly pristine weather of Thursday's exercise belies the harsh conditions that can often occur in the area, said David Holtz, chair of the nonprofit Sierra Club's Michigan chapter.

"What happens when the Straits freezes over, and there's up to several feet of ice?" he said. "Nothing I've heard from Enbridge, the Coast Guard or any of the other agencies involved suggests they have anything like a response in winter that is credible."

Keck said a winter oil spill response exercise at the Straits in February 2013 proved "very challenging." Winds can be severe, and temperatures can drop to around -20 degrees Fahrenheit in winter conditions, he said.

"The equipment wasn't robust enough to deal with those conditions," he said.

Enbridge plans another winter exercise at the Straits early next year, company spokesman Michael Barnes said.

Usher said if a spill coincided with rough waters and unsafe conditions for the company's response boats, "We'd have to wait. You can't put assets out on the water, and put lives and vessels at risk."

How quickly Enbridge and emergency agencies recognize a spill is occurring and begin a response can also be critical. After the company's Line 6B oil transmission line ruptured near Marshall In July 2010, Enbridge continued for 17 hours attempting to push through the line diluted bitumen — or dilbit — a heavy, sludgy oil mixed with other petroleum products. It resulted in the worst inland oil spill in U.S. history, devastating Talmadge Creek and a stretch of the Kalamazoo River, prompting a more than $1-billion cleanup that took more than four years.

Enbridge officials pointed to intense, constant scrutiny of Line 5, including underwater inspection of the pipe and evaluation of its integrity from within the line. Leak detection systems are designed to shut down the line within three minutes in the event of a pressure drop, Barnes said.

The Marshall spill "really shook us to the foundation," Enbridge President and CEO Al Monaco said. The years since have prompted "the largest integrity management and maintenance program in the history of pipelines" within the company, as well as enhanced monitoring and a change in culture and mind-set.

"If we think we have an issue, we are going to be cautious. We are going to be shut down," he said. "If we thought there was an issue to be operating Line 5, we would be shutting it down. I can assure you, we are doing everything in our power to operate it safely."

Enbridge Vice President Brad Shamla, Thursday's incident commander, oversaw ongoing activities from a command center set up at Little Bear Arena in St. Ignace.

"It's been going very well," he said, adding that he was thankful that the favorable weather allowed responders to deploy all of their response gear.

Participating agencies planned to debrief later Thursday, discussing what went well and where challenges existed during the exercise.

The nonprofit group For Love of Water on Monday released a report calling the Line 5 pipelines an "imminent harm" to the Great Lakes, and calling for the State of Michigan to take interim steps now to reduce the risk, including independent assessments of alternate pipeline routes, risk analysis, financial assurances and emergency response plans.

Michigan's Democratic U.S. Senators, Debbie Stabenow and Gary Peters, introduced a bill in Washington Wednesday that would ban crude oil shipments by sea vessels on the Great Lakes, and would call for a comprehensive review of Great Lakes pipelines and spill response plans.

Less than a mile from where Enbridge was conducting its exercises, at Bridge View Park, environmental groups and concerned citizens held a demonstration.

Phil Parr, a partner and general manager of Lake Charlevoix Brewing Co. in Charlevoix, noted that the community gets its drinking water from Lake Michigan, and that Michigan's clean water has been instrumental in its growing renown for micro-brewed beer.

"A spill would be detrimental to us and damaging to our community," he said.

One of two things is inevitable, said Aaron Payment, chairman of the Sault Ste. Marie Tribe of Chippewa Indians: Either the state acts to decommission Line 5 through the Straits of Mackinac before it fails, or it does so after an environmental catastrophe.

"Will we take the responsible action to avoid a tragedy?" he said. "Damage to our waters would be damage to our way of life. We will not accept that."

Enbridge spill factsFrom 2010 through 2014, Enbridge experienced 82 pipeline spills in 10 states, including the high-profile spill of more than 800,000 gallons from Line 6B near Marshall. Some facts:•Enbridge spilled 1,670,424 gallons of petroleum•Enbridge spills were 10.3% of the total U.S. gallons spilled•Enbridge spills caused 51% ($937 million) of the U.S. property damage total ($1.8B)Source: U.S. Pipeline and Hazardous Materials Safety Administration